Induction heat-treatment is a widely used process for the surface hardening of steel workpieces. The workpieces are heated by producing a high-frequency alternating magnetic field, so that selected surface regions of the workpiece are heated to a temperature within or above the transformation range, followed by immediate quenching. The core of the workpiece remains unaffected by this treatment and its physical properties are those of the bar from which it was machined, while the treated regions of the workpiece are metallurgically hardened.
One such workpiece is a bearing sleeve having an internal bore with bearing support surfaces or races disposed along the interior surface. Selected portions of the interior surface can be heat treated and metallurgically hardened by magnetic induction, in which an inductor body is positioned within the bore and quickly energized to magnetically induce an electric current in selected regions of the workpiece and heat those portions to a high temperature before quickly quenching them. The region of heat-treating of the interior surface of the workpiece is defined by the contour of the magnetic flux pattern produced by the coil of the inductor body.
The inductor body is connected to an AC power source adapted for this purpose, so that AC current flowing through the inductor will create a magnetic field that penetrates the workpiece and induces an eddy current in the workpiece. The heating of the workpiece by this eddy current and the subsequent quench is used to metallurgically harden the workpiece, but only the region in which the current is magnetically induced is hardened in this process. The other portions of the workpiece remain unaffected. The contour of the heating pattern is accomplished by the shape of the inductor and/or the shape of the coils on the inductor body.
In the case of heating the interior bore of a workpiece that has a varying inner diameter profile, such as a bearing sleeve, the induction element must have an outer diameter that is no larger than the smallest inner diameter of the workpiece bore, so that the induction element can be inserted and removed from the bore. A typical bearing sleeve for two sets of bearings has a bearing separator or straddle between the two bearing surfaces. At the straddle the interior bore has a reduced inner diameter, and the inductor body must have a maximum outer diameter no greater than this minimum inner diameter.
These limitations on the configuration of the inductor may cause the magnetic field produced by the inductor to heat portions of the workpiece that do not need to be heated. A further problem can result if these portions should not be hardened for various reasons, such as the need to perform further machining operations on these portions. For example, in the case of a bearing sleeve, it may not be possible using conventional induction elements to avoid hardening substantial portions of the straddle, and it may be desirable to perform further machining operations on the straddle, such as to drill a port through this portion of the bearing sleeve. If the straddle has been inductively hardened, it becomes more difficult to drill through the straddle.